Indoleamine-2,3-dioxygenase 1 (IDO1) is a heme-containing enzyme that catalyzes the oxidation of the indole ring of tryptophan to produce N-formyl kynurenine, which is rapidly and constitutively converted to kynurenine (Kyn) and a series of downstream metabolites. IDO1 is the rate limiting step of this kynurenine pathway of tryptophan metabolism and expression of IDO1 is inducible in the context of inflammation. Stimuli that induce IDO1 include viral or bacterial products, or inflammatory cytokines associated with infection, tumors, or sterile tissue damage. Kyn and several downstream metabolites are immunosuppressive: Kyn is antiproliferative and proapoptotic to T cells and NK cells (Munn, Shafizadeh et al. 1999, Frumento, Rotondo et al. 2002) while metabolites such as 3-hydroxy anthranilic acid (3-HAA) or the 3-HAA oxidative dimerization product cinnabarinic acid (CA) inhibit phagocyte function (Sekkai, Guittet et al. 1997), and induce the differentiation of immunosuppressive regulatory T cells (Treg) while inhibiting the differentiation of gut-protective IL-17 or IL-22-producing CD4+ T cells (Th17 and Th22) (Favre, Mold et al. 2010). IDO1 induction, among other mechanisms, is likely important in limiting immunopathology during active immune responses, in promoting the resolution of immune responses, and in promoting fetal tolerance. However in chronic settings, such as cancer, or chronic viral or bacterial infection, IDO1 activity prevents clearance of tumor or pathogen and if activity is systemic, IDO1 activity may result in systemic immune dysfunction (Boasso and Shearer 2008, Li, Huang et al. 2012). In addition to these immunomodulatory effects, metabolites of IDO1 such as Kyn and quinolinic acid are also known to be neurotoxic and are observed to be elevated in several conditions of neurological dysfunction and depression. As such, IDO1 is a therapeutic target for inhibition in a broad array of indications, such as to promote tumor clearance, enable clearance of intractable viral or bacterial infections, decrease systemic immune dysfunction manifest as persistent inflammation during HIV infection or immunosuppression during sepsis, and prevent or reverse neurological conditions.
IDO1 and Persistent Inflammation in HIV Infection:
Despite the success of antiretroviral therapy (ART) in suppressing HIV replication and decreasing the incidence of AIDS-related conditions, HIV-infected patients on ART have a higher incidence of non-AIDS morbidities and mortality than their uninfected peers. These non-AIDS conditions include cancer, cardiovascular disease, osteoporosis, liver disease, kidney disease, frailty, and neurocognitive dysfunction (Deeks 2011). Several studies indicate that non-AIDS morbidity/mortality is associated with persistent inflammation, which remains elevated in HIV-infected patients on ART as compared to peers (Deeks 2011). As such, it is hypothesized that persistent inflammation and immune dysfunction despite virologic suppression with ART is a cause of these non-AIDS-defining events (NADEs).
HIV infects and kills CD4+ T cells, with particular preference for cells like those CD4+ T cells that reside in the lymphoid tissues of the mucosal surfaces (Mattapallil, Douek et al. 2005). The loss of these cells combined with the inflammatory response to infection result in a perturbed relationship between the host and all pathogens, including HIV itself, but extending to pre-existing or acquired viral infections, fungal infections, and resident bacteria in the skin and mucosal surfaces. This dysfunctional host:pathogen relationship results in the over-reaction of the host to what would typically be minor problems as well as permitting the outgrowth of pathogens among the microbiota. The dysfunctional host:pathogen interaction therefore results in increased inflammation, which in turn leads to deeper dysfunction, driving a vicious cycle. As inflammation is thought to drive non-AIDS morbidity/mortality, the mechanisms governing the altered host:pathogen interaction are therapeutic targets.
IDO1 expression and activity are increased during untreated and treated HIV infection as well as in primate models of SIV infection (Boasso, Vaccari et al. 2007, Favre, Lederer et al. 2009, Byakwaga, Boum et al. 2014, Hunt, Sinclair et al. 2014, Tenorio, Zheng et al. 2014). IDO1 activity, as indicated by the ratio of plasma levels of enzyme substrate and product (Kyn/Tryp or K:T ratio), is associated with other markers of inflammation and is one of the strongest predictors of non-AIDS morbidity/mortality (Byakwaga, Boum et al. 2014, Hunt, Sinclair et al. 2014, Tenorio, Zheng et al. 2014). In addition, features consistent with the expected impact of increased IDO1 activity on the immune system are major features of HIV and SIV induced immune dysfunction, such as decreased T cell proliferative response to antigen and imbalance of Treg:Th17 in systemic and intestinal compartments (Favre, Lederer et al. 2009, Favre, Mold et al. 2010). As such, we and others hypothesize that IDO1 plays a role in driving the vicious cycle of immune dysfunction and inflammation associated with non-AIDS morbidity/mortality. Thus, we propose that inhibiting IDO1 will reduce inflammation and decrease the risk of NADEs in ART-suppressed HIV-infected persons.
IDO1 and Oncology
IDO expression can be detected in a number of human cancers (for example; melanoma, pancreatic, ovarian, AML, CRC, prostate and endometrial) and correlates with poor prognosis (Munn 2011). Multiple immunosuppressive roles have been ascribed to the action of IDO, including the induction of Treg differentiation and hyper-activation, suppression of Teff immune response, and decreased DC function, all of which impair immune recognition and promote tumor growth (Munn 2011). IDO expression in human brain tumors is correlated with reduced survival. Orthotropic and transgenic glioma mouse models demonstrate a correlation between reduced IDO expression and reduced Treg infiltration and a increased long term survival (Wainwright, Balyasnikova et al. 2012). In human melanoma a high proportion of tumors (33 of 36 cases) displayed elevated IDO suggesting an important role in establishing an immunosuppressive tumor microenvironment (TME) characterized by the expansion, activation and recruitment of MDSCs in a Treg-dependent manner (Holmgaard, Zamarin et al. 2015). Additionally, host IDO expressing immune cells have been identified in the draining lymph nodes and in the tumors themselves (Mellor and Munn 2004). Hence, both tumor and host-derived IDO are believed to contribute to the immune suppressed state of the TME.
The inhibition of IDO was one of the first small molecule drug strategies proposed for re-establishment of an immunogenic response to cancer (Mellor and Munn 2004). The d-enantiomer of 1-methyl tryptophan (D-1MTor indoximod) was the first IDO inhibitor to enter clinical trials. While this compound clearly does inhibit the activity of IDO, it is a very weak inhibitor of the isolated enzyme and the in vivo mechanism(s) of action for this compound are still being elucidated. Investigators at Incyte optimized a hit compound obtained from a screening process into a potent and selective inhibitor with sufficient oral exposure to demonstrate a delay in tumor growth in a mouse melanoma model (Yue, Douty et al. 2009). Further development of this series led to INCB204360 which is a highly selective for inhibition of IDO-1 over IDO-2 and TDO in cell lines transiently transfected with either human or mouse enzymes (Liu, Shin et al. 2010). Similar potency was seen for cell lines and primary human tumors which endogenously express IDO1 (IC50s˜3-20 nM). When tested in co-culture of DCs and naïve CD4+CD25− T cells, INCB204360 blocked the conversion of these T cells into CD4+FoxP3+ Tregs. Finally, when tested in a syngeneic model (PAN02 pancreatic cells) in immunocompetent mice, orally dosed INCB204360 provided a significant dose-dependent inhibition of tumor growth, but was without effect against the same tumor implanted in immune-deficient mice. Additional studies by the same investigators have shown a correlation of the inhibition of IDO1 with the suppression of systemic kynurenine levels and inhibition of tumor growth in an additional syngeneic tumor model in immunocompetent mice. Based upon these preclinical studies, INCB24360 entered clinical trials for the treatment of metastatic melanoma (Beatty, O'Dwyer et al. 2013).
In light of the importance of the catabolism of tryptophan in the maintenance of immune suppression, it is not surprising that overexpression of a second tryptophan metabolizing enzyme, TDO2, by multiple solid tumors (for example, bladder and liver carcinomas, melanomas) has also been detected. A survey of 104 human cell lines revealed 20/104 with TDO expression, 17/104 with IDO1 and 16/104 expressing both (Pilotte, Larrieu et al. 2012). Similar to the inhibition of IDO1, the selective inhibition of TDO2 is effective in reversing immune resistance in tumors overexpressing TDO2 (Pilotte, Larrieu et al. 2012). These results support TDO2 inhibition and/or dual TDO2/IDO1 inhibition as a viable therapeutic strategy to improve immune function.
Multiple pre-clinical studies have demonstrated significant, even synergistic, value in combining IDO-1 inhibitors in combination with T cell checkpoint modulating mAbs to CTLA-4, PD-1, and GITR. In each case, both efficacy and related PD aspects of improved immune activity/function were observed in these studies across a variety of murine models (Balachandran, Cavnar et al. 2011, Holmgaard, Zamarin et al. 2013, M. Mautino 2014, Wainwright, Chang et al. 2014). The Incyte IDO1 inhibitor (INCB204360, epacadostat) has been clinically tested in combination with a CTLA4 blocker (ipilimumab), but it is unclear that an effective dose was achieved due to dose-limited adverse events seen with the combination. In contrast recently released data for an on-going trial combining epacadostat with Merck's PD-1 mAb (pembrolizumab) demonstrated improved tolerability of the combination allowing for higher doses of the IDO1 inhibitor. There have been several clinical responses across various tumor types which is encouraging. However, it is not yet known if this combination is an improvement over the single agent activity of pembrolizumab (Gangadhar, Hamid et al. 2015). Similarly, Roche/Genentech are advancing NGL919/GDC-0919 in combination with both mAbs for PD-L1 (MPDL3280A, Atezo) and OX-40 following the recent completion of a phase 1a safety and PK/PD study in patients with advanced tumors.
IDO1 and Chronic Infections
IDO1 activity generates kynurenine pathway metabolites such as Kyn and 3-HAA that impair at least T cell, NK cell, and macrophage activity (Munn, Shafizadeh et al. 1999, Frumento, Rotondo et al. 2002) (Sekkai, Guittet et al. 1997, Favre, Mold et al. 2010). Kyn levels or the Kyn/Tryp ratio are elevated in the setting of chronic HIV infection (Byakwaga, Boum et al. 2014, Hunt, Sinclair et al. 2014, Tenorio, Zheng et al. 2014), HBV infection (Chen, Li et al. 2009), HCV infection (Larrea, Riezu-Boj et al. 2007, Asghar, Ashiq et al. 2015), and TB infection (Suzuki, Suda et al. 2012) and are associated with antigen-specific T cell dysfunction (Boasso, Herbeuval et al. 2007, Boasso, Hardy et al. 2008, Loughman and Hunstad 2012, Ito, Ando et al. 2014, Lepiller, Soulier et al. 2015). As such, it is thought that in these cases of chronic infection, IDO1-mediated inhibition of the pathogen-specific T cell response plays a role in the persistence of infection, and that inhibition of IDO1 may have a benefit in promoting clearance and resolution of infection.
IDO1 and Sepsis
IDO1 expression and activity are observed to be elevated during sepsis and the degree of Kyn or Kyn/Tryp elevation corresponded to increased disease severity, including mortality (Tattevin, Monnier et al. 2010, Darcy, Davis et al. 2011). In animal models, blockade of IDO1 or IDO1 genetic knockouts protected mice from lethal doses of LPS or from mortality in the cecal ligation/puncture model (Jung, Lee et al. 2009, Hoshi, Osawa et al. 2014). Sepsis is characterized by an immunosuppressive phase in severe cases (Hotchkiss, Monneret et al. 2013), potentially indicating a role for IDO1 as a mediator of immune dysfunction, and indicating that pharmacologic inhibition of IDO1 may provide a clinical benefit in sepsis.
IDO1 and Neurological Disorders
In addition to immunologic settings, IDO1 activity is also linked to disease in neurological settings (reviewed in Lovelace Neuropharmacology 2016(Lovelace, Varney et al. 2016)). Kynurenine pathway metabolites such as 3-hydroxykynurenine and quinolinic acid are neurotoxic, but are balanced by alternative metabolites kynurenic acid or picolinic acid, which are neuroprotective. Neurodegenerative and psychiatric disorders in which kynurenine pathway metabolites have been demonstrated to be associated with disease include multiple sclerosis, motor neuron disorders such as amyotrophic lateral sclerosis, Huntington's disease, Parkinson's disease, Alzheimer's disease, major depressive disorder, schizophrenia, anorexia (Lovelace, Varney et al. 2016). Animal models of neurological disease have shown some impact of weak IDO1 inhibitors such as 1-methyltryptophan on disease, indicating that IDO1 inhibition may provide clinical benefit in prevention or treatment of neurological and psychiatric disorders.
It would therefore be an advance in the art to discover IDO inhibitors that effective the balance of the aforementioned properties as a disease modifying therapy in chronic HIV infections to decrease the incidence of non-AIDS morbidity/mortality; and/or a disease modifying therapy to prevent mortality in sepsis; and/or an immunotherapy to enhance the immune response to HIV, HBV, HCV and other chronic viral infections, chronic bacterial infections, chronic fungal infections, and to tumors; and/or for the treatment of depression or other neurological/neuropsychiatric disorders.    Asghar, K., M. T. Ashiq, B. Zulfiqar, A. Mahroo, K. Nasir and S. Murad (2015). “Indoleamine 2,3-dioxygenase expression and activity in patients with hepatitis C virus-induced liver cirrhosis.” Exp Ther Med 9(3): 901-904.    Balachandran, V. P., M. J. Cavnar, S. Zeng, Z. M. Bamboat, L. M. Ocuin, H. Obaid, E. C. Sorenson, R. Popow, C. Ariyan, F. Rossi, P. Besmer, T. Guo, C. R. Antonescu, T. Taguchi, J. Yuan, J. 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